The two previous installments in this series described problems in concrete due to faulty concreting practices and some difficulties which can arise due to external influences on hardened concrete. This installment will conclude the series with discussions of several more problems due to external influences. Loss of bond can occur after the concrete has hardened, or bond may never have been achieved. The most common cause for bond failures is insufficient or improper preparation to the hardened concrete against which the plastic concrete is to be placed. To avoid loss of bond: remove loose material from hardened concrete; slightly roughen hardened concrete surface; avoid laitance on surfaces to be bonded; take required steps to develop high strength at the plane along which bond is to be developed; and cure thoroughly. There are some materials that will corrode when embedded in concrete. Aluminum, and especially aluminum alloys, have been known to corrode in concrete, although the precise reason for this corrosion is matter of debate. If the concrete is kept dry, little or no corrosion should take place; however, this is usually a difficult state to maintain in concrete. If aluminum equipment is to be buried in concrete, it is probably best to avoid calcium chloride as an admixture. Electrolytic action in the concrete will also cause corrosion. The list of materials which attack concrete is long and involved. The various aggressive materials attack different components of the concrete mass and in varying degrees. The subject is too extensive to cover here. A comprehensive discussion of the subject appeared in an article entitled, "Agecies that Attack Concrete" in the December, 1957, issue of Concrete Construction. By erosion we refer to the deteriorating effect of rapidly moving water containing suspended solids. Cavitation is defined as deterioration of concrete caused by turbulence in the flow of water over or around concrete. Although the most effective way to prevent cavitation and erosion is through proper design, these are some cases where it is impossible to avoid turbulence. Field test have shown that the use of pozzolans in lean mass concrete appreciably increases its resistance to cavitation. The most effective treatment, however, is to apply a layer of rubber or metal over the surface to be exposed. This article also discusses the effects of efflorescence and fire and possible solutions.